Due to the influence of the acoustic stiffness inherent to a hollow chamber of a speaker cabinet, it is generally difficult to realize a speaker system which is capable of reproducing a low frequency sound using a compact-size loudspeaker system. In an attempt to realize reproduction of a low frequency sound with a compact-size loudspeaker system, an airtight loudspeaker system whose cabinet houses a lump of activated carbon inside is known as means which solves the problem that the capacity of a cabinet sets a limit upon reproduction of a low frequency sound. (See Patent Document 1 for instance.)
FIG. 10 is a cross sectional view which shows the structure of an essential part of the loudspeaker system described in Patent Document 1 mentioned above. In FIG. 10, the loudspeaker system comprises a cabinet 101, a low frequency sound speaker 102, activated carbon 103, a support member 104, a diaphragm 105 and a vent pipe 106. The low frequency sound speaker 102 is fitted to the front surface of the cabinet 101. The activated carbon 103 is disposed as a lump inside the cabinet 101 and supported by the back surface, the bottom surface, the top surface, the side surfaces of the right-hand side and the left-hand side of the cabinet 101 and by the support member 104. All over the surface of the support member 104, there are fine pores which permit air ventilation. The vent pipe 106 is disposed to the diaphragm 105 and ventilates between the activated carbon 103 and the low frequency sound speaker 102.
An operation of the loudspeaker system above will now be described. Application of an electric signal upon the low frequency sound speaker 102 changes the pressure inside the cabinet 101, and this pressure vibrates the diaphragm 105. As the diaphragm 105 vibrates, the pressure inside the hollow chamber in which the activated carbon 103 is disposed changes. While the support member 104 and the cabinet 101 support the activated carbon 103 as a lump, due to the fine pores formed all over the surface of the support member 104, the change of the pressure attributable to the vibrations of the diaphragm 105 causes absorption of air molecules by the activated carbon 103, whereby a variation of the pressure inside the cabinet 101 is suppressed.
In the conventional loudspeaker system, the cabinet 101 thus operates equivalently as a large-capacity cabinet, i.e., although being a compact-size cabinet, reproduces a low frequency sound as if a loudspeaker unit were mounted to a large cabinet. Meanwhile, the vent pipe 106, owing to the temperature around the loudspeaker system and a pressure change inside the loudspeaker system, prevents a pressure variation within the space which houses the activated carbon 103 and is surrounded by the diaphragm 105 and the cabinet 101.
In the meantime, a bass reflex-type speaker cabinet is generally used as a system which enhances a low frequency sound better than an airtight cabinet does. FIG. 11 is a cross sectional view which shows the structure of a bass reflex-type loudspeaker system. In FIG. 11, the illustrated loudspeaker system comprises a cabinet 111, a loudspeaker unit 112 and an acoustic port 113. The loudspeaker unit 112 is fitted to the front surface of the cabinet 111. The acoustic port 113 is disposed to the front surface of the cabinet 111 so that via the acoustic port 113, a hollow chamber R defined by the cabinet 111 is opened to outside. Utilizing the acoustic capacity of the cabinet 111 and acoustic resonance due to the acoustic port 113 disposed to the cabinet 111, the loudspeaker system radiates a low frequency sound.                [Patent Document 1] Published Patent Application No. 60-500645        